In the past two decades, tobacco expansion processes have become an important part of the cigarette manufacturing process. Tobacco expansion processes are used to restore tobacco bulk density and/or volume which are lost during curing and storage of tobacco leaf. In addition, expanded tobacco is an important component of many low tar and ultra-low tar cigarettes.
U.S. Pat. No. 3,524,451 to Fredrickson and U.S. Pat. No. 3,524,452 to Moser et al. describe processes in which tobacco is contacted with an impregnant and then heated rapidly to volatilize the impregnant and expand the tobacco. U.S. Pat. No. 3,683,937 to Fredrickson et al. discloses the vapor state impregnation of tobacco followed either by heating or rapid pressure reduction for tobacco expansion.
The use of a carbon dioxide for expanding tobacco is disclosed in U.S. Pat. No. 4,235,250 to Utsch; U.S. Pat. No. 4,258,729 to Burde et al.; and U.S. Pat. No. 4,336,814 to Sykes et al., among others. In these and related processes, carbon dioxide, either in gas or liquid form, is contacted with tobacco for impregnation and the impregnated tobacco is subjected to rapid heating conditions for expansion. In the known carbon dioxide expansion processes, it is typically necessary to heat the tobacco excessively in order to achieve substantial and stable expansion. This excessive heating can harm the tobacco flavor and/or generate an excessive amount of tobacco fines. In addition, those processes which use liquid carbon dioxide for impregnating tobacco typically result in impregnated tobacco in the form of solid blocks of tobacco containing dry ice, which must be broken up prior to heat treatment, thereby increasing the complexity of the process.
U.S. Pat. No. 4,388,932 to Merritt et al. discloses a process for increasing the post-reordering filling capacity of previously expanded tobacco. Previously expanded tobacco having an `Oven Volatiles` (OV) content of less than 6 percent is heated to reduce its OV content to a value said to be well below 3 percent. The OV content of tobacco is said to be approximately equivalent to its moisture content since no more than 0.9 percent of tobacco weight is volatiles other than water. The very low OV content tobacco recovered from the post-expansion heating step is subjected to a reordering step for increasing its moisture content and is said to collapse less during the reordering step than if it were not heat treated after expansion. A stiffening of the tobacco during the heat treatment was proposed to account for the increased stability of the expanded tobacco during reordering.
U.S. Pat. No. 4,531,529 to White and Conrad describes a process for increasing the filling capacity of tobacco, wherein the tobacco is impregnated with a low-boiling and highly volatile expansion agent, such as a normally gaseous halocarbon or hydrocarbon at process conditions above or near the critical pressure and temperature of the expansion agent. The pressure is quickly released to the atmosphere so that the tobacco expands without the necessity of a heating step to either expand the tobacco or fix the tobacco in the expanded condition. The pressure conditions of this process range from 36 Kg/cm.sup.2 (512 psi) and higher with no known upper limit. Pressures below 142 Kg/cm.sup.2 (2,000 psi) were used to produce satisfactory tobacco expansion without excessive fracturing.
U.S. Pat. No. 4,554,932 to Conrad and White describes a fluid pressure treating apparatus, including a cylindrical tubular shell and a spool assembly mounted for reciprocal movement between a loading position outside the shell and a treating position within the shell. When the spool is within the shell, deformable sealing rings carried in annular grooves on the cylindrical ends of the spool are forced radially outwardly for engagement with the interior of the shell to form a pressure chamber within the shell between the spool ends. Conduits are provided to introduce processing fluids into the annular pressure chamber formed within the shell. The use of this apparatus for high pressure impregnation of tobacco with an expansion agent permits easy loading and unloading of tobacco and avoids the closure and opening problems associated with conventional pressure sealing and locking mechanisms, such as pivotable autoclave lids. This pressure vessel can thus produce time savings and improve economics in tobacco expansion.
Tobacco expansion processes including those described above and others, must be conducted in batch processes when impregnation pressures substantially above atmospheric pressure are used. The batch treating processes require complicated treating apparatus and long cycle times because of the time required in opening and closing the vessels and introducing and removing impregnating agent from the vessels. Some throughput improvements have been made by modifying the various apparatus employed to decrease cycle time; however, substantial throughput improvements in the known batch systems are available according to conventional techniques primarily by increasing volumes of the individual systems and/or increasing the number of batch systems used simultaneously.